Process for manufacturing electron emissive material and electrodes



y 4, 1954 P. DELRIEU ET AL 2,677,623

PROCESS FOR MANUFACTURING ELECTRON EMISSIVE MATERIAL. AND ELECTRODESFiled Oct. 25, 1950 2 Sheets-Sheet 1 Invenfm s Pierre DeZm'eu 1* Andre.czl uaze May 4, 1954 P. DELRIEU ET AL 2,677,623 PROCESS FORMANUFACTURING ELECTRON EMISSIVE MATERIAL AND ELECTRODES Filed Oct. 23.1950 2 Sheets-Sheet 2 Fig.3

WWW/mm WMYMWMMM In venzm- Pie/we DeZ/1'eu 1- A vzcZre CZauaZe.

. Alia .53

Patented May 4, 1954 UNITED STATES PATENT OFFICE PROCESS FORMANUFACTURING ELECTRON EMISSIVE MATERIAL AND ELECTRODES French companyApplication October 23, 1950, Serial No. 191,572

Claims priority, application France October 27, 1M9

'7 Claims. 1

This invention relates to electrodes for electrical discharge apparatus,specially those having a gas atmosphere, and to processes for makingsuch electrodes.

The electrodes used for such apparatus are generally classified into twogroups: so called cold electrodes, the temperature of which generallydoes not exceed anywhere that of a dull red heat, and so-called hotelectrodes, which, in operation, generally locally attain a white redheat, or are even hotter.

Cold electrodes have a long operating life, but they occasion a largevoltage drop (cathode+anode voltage drops): 100 to 250 v.

Hot electrodes have a shorter operating life and deteriorate morerapidly on account of startings, particularly of instantaneous startingsbut they have a smaller voltage drop: 15 to 25 volts. Further, thedischarge current of a hot electrode should be within fairly narrowlimits, for instance between 0.3 and 0.6 amp. if a normal life isdesired for this electrode.

One object of this invention is to provide an electrode havingsimultaneously a long operating life, and a low voltage drop.

Another object is to provide an electrode of which the discharge currentmay vary between a maximum and one fifth or one tenth of said maximum.

A further object of the invention is to provide an electrode of smallvolume.

A still further object of the electrode is to provide an electrodecomprising an emissive material obtained by reacting barium dioxide withmolybdenum r tantalum or with both these metals, at a temperature above525 C. and lower than about 1,30i) C., said reaction taking place, atleast at the end of its duration, in a nonoxidizing atmosphere. Saidreaction may be effected, very plainly, by melting the barium dioxide incontact with the molybdenum or tantalum or with both these metals. Theterm ncnoxidizing atmosphere comprises inert atmospheres as well asreducing ones.

Better results are obtained when the emissive coating is thick than whenit is relatively thin.

It is generally not useful to fix the activating substances to itssupport by means of sintering; the plain melting of the barium dioxide,for instance, causes indeed this dioxide, as well as the substancesyielded by its reaction on molybdenum and tantalum and by itsdecomposition, to hold in a very satisfactory way on supports of variouskinds. The activating substance may thus form a coating which is thick,tough and strong, on

2 the support which allows the so devised electrode to work during avery long time and to stand to very numerous startings withoutpreheating, while offering only a low voltage drop.

Further features and advantages of this invention will appear from thefollowing detailed description of species thereof. For a betterunderstanding of this invention reference may be had to the followingdescription taken in connection with the accompanying drawings and itsscope will be pointed out in the appending claims.

Figure 1 is a view, in elevation, of a portion of a discharge apparatushaving a mercury and rare gas atmosphere, particularly argon, krypton,xenon;

Figure 2 is a section through the axis of the electrode of the apparatusshown in Figure 1;

Figure 3 shows, in partly sectioned elevation, another electrode withits support acting as a current supply lead, the extremity of the sternof discharge apparatus to which this electrode is secured, and theadjacent portion of the envelope of this apparatus;

Figure 4 shows an electrode according to a further modification;

Figure 5 shows an electrode according to yet another modification;

Figure 6 is a graph showing the results of tests which have beeneffected on a discharge device provided with electrodes constructedaccording to the invention.

The electrode shown in Figures 1 and 2 has a hot zone and a cold zone,the electrode comprises a wall, formed by welding together a sleeve Ihaving an inwardly directed rib at the lower end thereof, a disc 2placed over the rib and serving to close the lower end or" the sleeveand a cup element 3 placed above the disc 2; the edges of said cupelement touch the sleeve or at least are very close to it so as toprevent activating material from flowing out of the electrode duringmanu facture thereof. Parts i, 2, 3 are made of molybdenum sheet 0.1 mm.thick; they form a cavity, the inner wall of which is covered with arelatively thick layer t of emissive materials, said layer beingobtained, for instance, in the maner explained hereinafter.

The Wall E, 2, 3 is welded to a nickel yoke 5 which connects said wallto the current supply leads 8, 9 of the electrode and, in addition,serves to cool the lower portion of said electrode during operation ofthe apparatus, thus creating a cold Zone at the lower end of theelectrode.

A cap 6, of insulating material, for example steatite, covers thegreater portion of the outer surface of the molybdenum wall, leaving thelower end and an annular area close to said lower end uncovered; the-cap6 also coVers-theupper edge of the sleeve l and protrudes a littleinside the latter. This can prevents electrical discharges from takingplace from the portions of the wall surfaces it covers and, in addition,insulates these portions against heat loss, whereby a"hotzone is createdat the upper end of the wall during operation of the apparatus. The caphas an aperture I through which the discharges pass, and thus haveaccess to the inside'of the electrode. This cap is slipped on themolybdenum wall and is held by the current supply leads 8 and 9 whichare bent over the flared portion ll! of the cap.

To prevent discharges starting from anywhere else than from inside theelectrode, the portions of the outer surface of the Wall of the latterwhich are not covered by the cap a are coated for instance with analumina anti-emissive layer ll. This layer is obtained, for example, bycoating these surfaces, by brushing them, or by spraying them, withfinely divided alumina in suspension in an organic liquid, ethyl alcoholfor instance. The current supply leads 8, 9 and the yoke dare coatedwith the same kind of layer. This ensures that neither these parts northe outer surface of the lower end of the molybdenum wall nor the outersurface of the annular zone close to said lower end are heated byoperating as anodes or cathodes. This helps in maintaining the cold zonealready referred to; the triple thickness of the lower end of the wallalso assists in maintaining said cold zone. It is to be understood thatdischarges could also be prevented from taking place from the outersurface of the electrode or from the members connected to the electrodeby threading the cap 6 with a mica washer, the periphery of which willbe near the envelope I8 inside which the electrode will be mounted.

As is usual in the luminescent tube industry, the electrode is mountedon a glass stem l2, pinched on its current supply leads; 8, Q, and whichmay comprise an exhaust tube i3 as in the case shown in Figure l.

The following method may be used for obtaining the layer 4' of emissivematerials; First, there is placed in the cavity constituted by the wallE, 2, 3, activating material, barium dioxide powder in the present case,then this material is melted by heating for example'with a torch; themelt forms a layer of activating material covering the inside of themolybdenum wall. It may be advantageous (to make later operations morerapid) to then heat the electrode around 850 C., for half an hour in aninert gas such as nitrogen, or a reducing gas such as hydrogen. Theformation of the electrode and the corresponding operation. for theother electrode of the discharge apparatus are then eii'ected, forinstance. as is usual in this technique, by mounting these electrodesinside the apparatus, evacuating the latter and introducing therein arare gas. for instance argon, under a pressure of a few millimeters ofmercury, then arcing said apparatus.

The electrode shown in Figure 3 comprises a molybdenum wall mm. inthickness, constituted by a cylinder 2! and an end portion 22. Thiscylinder is welded to a nickel wire 24, 0.8 mm. in diameter, forinstance, which supports the electrode and supplies it with current. Thewire 2., is sealed tightly in a glass stem 26, itself welded to theenvelope 21. The molybdenum wall, before being mounted in the envelope,is preferably subjected to a heating at about 1200 (3., in

a hydrogen atmosphere, which, amongst other things, eliminates alltraces of oxidation.

Thewell of theelectrodeis coated, on its inner face, with arelativelythick coating 23 of emissive materials, obtained as follows: First, thecavity limited by the wall 2|, 22, is filled with barium dioxide, pureor mixed with other materials. The-whole is then heated, at least to ared heat, in a reducing atmosphere consisting, for instance, of amixtureof nitrogen and hydrogen, or of ammonia gas;. the duration of thisheating should be at least equal to the period at the end of which thebarium dioxide has become greyblack, the production of metallic bariumnot being necessary: if the heating is effected at around 1250" (3.,this duration is of the order of one half minute, i., e. relativelyshort. This heating may be obtained by inducing in the wall 2 I 22, ahigh frequency electric current, by means of a coil such as thatrepresented at 28 in Figure 4. The purpose of this heating operation isto cause the melting of the barium dioxide and to cause a mutualreaction of the barium dioxide and molybdenum. The product obtainedgenerally contains about 73% barium, 7% molybdenum and 2t% oxygen,which-corresponds approximately to the formula 'iBtOz-l-lMoOs; thisformula is given only as representing the percentage composition of thematerial as it was found impossible to determine-of what mixture ofmaterials it consists or whether it is composed of a definite productfor the major part and, for the rest, of small amounts of products fromother stages of the reaction.

This heating may be effected simply in a nonoxidizing atmosphere(nitrogen; rare gas), but it is preferably that this atmosphere bereducing, at least during the end of the reaction; in practice it issimpler to use a reducing atmosphere during the whole duration of theheating. The above reaction takes place fairly rapidly but generallydoes not-cause an overflow of the materials contained in the cavity. Anoverflow would be; e the troublesome effect that the outer face of theelectrode wall would be coated, at least locally, with emissivematerials.

In the operation such as effected in practice and which takes place inan atmosphere of nitrogen and hydrogen, the high frequency 1.0 firstcauses a gradual rise, inside about i seconds, of the temperature of thewall to 900 or 1006 G. Then the reaction starts, which is shown by asudden temperature rise of the wall and by seething of the materialscontained in the cavity of the electrode; within about one second, thetemperature of the wall rises to about 7.368 C. and then falls ofi toabout 1100 C. The reaction does not take place violently. When theseething ceases, the reaction is practically ended but the heating isstill kept up for about four seconds, to keep the temperature aroundll00 to be sure that the reaction is complete. From the beginning of theheating, about 9 seconds have elapsed. The material contained in theelectrode cavity has then a grey-black colour. It is generallyunnecessary to continue the reaction to the point where metallic bariumis given off by the emissive materials and volatilizes therefrom, atthat stage, in an appreciable amount. The electrode is then allowed tocool for 10 or 12 seconds in the atmosphere where the remainder of thetreatment was effected. All siu faces, except the inside of the cavity,where a cathode spot could occur are then covered with an insulatingdeposit of alumina 25, i. e, the outer surface of, the wall 2!. 22

and the current supply lead 23. This deposit 25 is obtained, forinstance, by spraying with a spray-gun, a suspension of powdered aluminain alcohol; it may without any inconvenience, also cover a portion ofthe stem 26.

A replacement of the dioxide by another bariurn oxide or a bariumhydrate gives poor results; another oxide would not react withmolybdenum, a hydrate would not react either and in some cases wouldcause troublesome overflows during the reaction.

If a similar process is attempted for the reaction of barium dioxidewith tungsten, the reac tion is violent and even somewhat explosive. Thegreater part of the materials is expelled from the electrode cavity andwhat remains does not stick to the walls; adherence, however, is a majorrequirement when the electrode is to operate at a high temperaturewithout being pre-heated before starting discharges. The reaction isalso too violent when barium dioxide is in contact with a too stronglyreducing agent, such as aluminium or carbon.

The emissive deposit obtained by the above process, however, forms avery adherent and dense crust through no sintering is effected. Thisdeposit is little damaged by remaining in air a short time, contrary tooxides or hydrates which are often used. Thus the electrodes can behandied without any special precautions during later operations.

Two electrodes thus obtained my immediately be placed in position bywelding of the stems to which they are secured, respectively to the twoends of a glass tube, coated or not with fluorescent materials. Thewhole assembly is then subjected to the usual operations for themanufacturing of fluorescent lamps and similar discharge apparatus:gases are eliminated by the usual methods (heating effected in a vacuum,in an oven or by discharges, heating of the electrodes by a highfrequency field) the filling with rare gases is effected, a drop ofmercury is introduced and the device is operated for a few minutes tocause a diffusion of the mercury inside the lamp.

It should be noted that, generally, the gas removing operation is nolonger accompanied by the formation of the electrode and has not to beeffected any longer by means or" continued discharges of a highintensity; the electrode is in an activated condition as soon as thereaction between the barium dioxide and the molybdenum is ended.

If it is desired to store an electrode for some time before mounting it,it will be suflicient to pour one drop of collodion in its cavity. Whendrying, this drop leaves on the surface of the emissive materials atight film of nitrogencellulose; this film will then disappear duringthe gas removal in the tube inside which the electrode will be mounted.

It seems probable that during the operation of this electrode the bariumdioxide continues to react on the molybdenum of the wall; observationshows that the latter is gradually corroded and is eventually piercedthrough.

It may also be observed that the stains which, after a certain time ofoperation, appear in the vicinity of the electrodes, contain somemolybdenum; on the other hand, contrary to the stains due to bariumactivated electrodes obtained by known methods, it is often impossibleusing electrodes as described above to detect any barium and, in anycase, the amount of barium is so 6 small that it cannot be determined byusual chemical methods.

Such an electrode, for giving out a current which may vary from 0.1 to0.5 ampere may have, for instance, a diameter of 4 mm. and a length of10 mm; the corresponding amount of barium dioxide used for itsactivation is of the order of milligrams. This electrode, operating at0.4 amp. may undergo at least 200,000 startings before being put out ofuse, each one of said startings being followed by an operating period of10 seconds and a pause of 10 seconds. This corresponds to a probablenormal use life, or the order of 15,000 hours. The sum of the anodevoltage drop and of the cathode voltage drop, in operation, is about 20to 40 volts for currents of 0.2 to 0.5 amp. The hot zone of thiselectrode is a very small area, almost a point, the temperature ofwhich, in the case of an electrode having the dimensions indicated,varies from 1200 to 1800 0., according to the current (0.1 to 0.6 amp).In the immediate vicinity of this area, the temperature drops veryrapidly, for instance to 700 C. for a current of 0.4 amp. or 900 C. fora current of 0.6 amp; the temperature drops still more on getting closerto the place where the current supply lead is secured.

The volume of this electrode, which is only 0.12 cm for a normal maximumcurrent of 0.5 amp., is much smaller than the volume of usualnon-filament electrodes of the same power. In this volume, "0.1 gram ofbarium dioxide may be introduced without any trouble due to overflowingduring the reaction or the heating which precedes it.

The electrode shown in Figure 4. is similar to that shown in Figure 3;its wall, however, 2Q, is not made of molybdenum, but of very pure iron,metallic molybdenum being supplied by a wire 30 of this metal, helicallywound, and inserted within the cavity before the barium dioxide isintroduced therein. The electrode is supported by two lead wires SI, 32.

This same Figure 4 shows, in section, the coil 28 inside which theelectrode is placed with a view to heating the barium dioxide andcausing its reaction with the molybdenum. When a current of suitablefrequency and intensity flows through said coil, it induces in theelectrode wall, and, in the present case, in the molybdenum helix, 30,eddy currents which heat up these metal parts and, by thermalconduction, the barium dioxide and other materials which may becontained in the electrode cavity.

Figure 5 shows an electrode wherein the wall. 33 is made of ceramicmaterial, steatite for instance. The electric current is supplied by thelead 34 which extends into a sort of helix 35, within the emissivematerial 23. The metallic molybdenum used in the make-up of the latteris introduced inside the electrode cavity, for instance in the form ofmolybdenum grains, roughly mixed, previously, with the barium dioxide. Awasher 36, made of molybdenum or iron, for instance, prevents the liquidbarium dioxide from flowing through the clearance between the lead 34and the aperture of the wall 33 through which said lead enters theelectrode cavity.

On Figures 3 and 4, the aperture in the electrode is directed towardsthe stem to which the latter is secured, so that the black stainsgenerally observed in the vicinity of the electrodes occur near thestem; in Figure 5, on the corn trary, the aperture in the electrode isdirected 7' in the opposite direction. It is obvious that one or theother of these directions may be used indifferently with the three typesof electrodes shown in Figures 3, 4, 5. Similarly, each eiectrode may beprovided with one or two supports, or even more, one or several of saidsupports possibly not being used as lead wires. The metallic molybdenumfor each one of the electro es may be in the form of sheets, wire,powder, etc. or several such forms simultaneously.

Numerous other modifications may be made to the electrodes described,within the scope of the invention; in particular, the reaction betweenthe barium dioxide and molybdenum may be effected before mounting theelectrode on its stem, the materials generating the emissive substancesmay contain other materials than on rium dioxide and molybdenum: forinstance strontium dioxide, molybdeniun compounds, thorium or tungsten,metallic or in compounds, silica, etc.; the amount of those of saidother materials which contain alkaline-earth metal must, however, besmall relatively to the amount of barium dioxide.

The reaction of the barium dioxide may also be effected on tantalum and,optionally, molybdenum.

The electrode may, then, be again, for example, of the type shown inFigure 3, its wall eomprising a cylindrical ferrule and a closed end,both made out of a molybdenum sheet 0.1 mm. thick. This wall isde-oxidized by heating to about 1200 O. in hydrogen, then welded to acurrent lead, itself sealed in a stem. The activating coating of theelectrode is obtained as follows. There is introduced into the cavity ofthe wall, a ture of powdered barium dioxide and metallic tantalumpowder, a mixture which has been left in damp air for one day. Theelectrode, attached to its support, is then installed inside a chamberthrough which a mixture of nitrogen and hydrogen is passed. Then, bymeans of a high frequency magnetic field, the wall of the electrode israised to above red heat temperature during a time sufiicient to obtaina melting or". the barium dioxide and its reaction on the molybdenum.This stage in the manufacture was described above in detail inconnection with Figure 3. In the present case, tantalum is in contactwith the dioxide and reacts with the latter. This does not altersubstantially the term perature and time of reaction, which is, forinstance, about ten seconds at about 1200 C.

The inside of the electrode is, when the reaction is complete, coveredwith an adherent crust, granite-like and blackish, which constitutes theemissive substance activating the electrode. The electrode is allowed tocool for about ten seconds inside the chamber, then it is removed andits outer surface and connecting lead are coated with alumina.

An electrode thus manufactured has little tendency to give rise to blackstains on the envelope of the lamp in which it is mounted; moreover, itcan be left several minutes in the open air without any inconvenientdeterioration.

Such an electrode, intended for utilizing a current varying from 30 to250 milliamperes, may have, for example, a diameter of 4 mm. and alength of 5 mm.; the corresponding amounts of barium dioxide andtantalum which are used are respectively of the order of 60 milligramsand 40 milligrams.

These amounts, or even their proportions may also vary largely whilegiving good results. The

8 tantalum may be used in forms other than a powder, for example in theform of a wire or flakes. It may even constitute all or part of the wallof the electrode. The tantalum may be completely submerged in the bariumdioxide but it may also, if used in the form of a wire, for instance,rise abcve the surface of the dioxide and even protrude out of thecavity of the electrode.

The wall of the electrode may consist neither of molybdenum nor oftantalum, but for instance of very pure iron or ceramic, as abovementioned; it is not necessary that metal molybdenum be in contact withthe barium dioxide during the reaction and furthermore there may be nomolybdenum in the electrode.

The atmosphere in which the melting is effected may be different from amixture of hydrogen and nitrogen and may be, for instance, hydrogen,ammonia gas, nitrogen, a rare gas etc., but it is generally preferablethat the reaction of the barium dioxide with the tantalum and, as thecase may be, with the molybdenum, end in a reducing atmosphere. Water,the presence of which may not be necessary, may be introduced with oneonly of the materials used for producing the emissive substances, forinstance by slightly humidifying the barium dioxide, or it may be addedas a very small water drop to these materials after they are placed inposition in the cavity of the electrode, but before the reaction, or byusing a damp gas for constituting the atmosphere in which this reactionis effected.

The reaction of barium dioxide, at the time of its melting, takes placewith a troublesome rapidity in some cases, particularly when thismaterial is very pure and dry. This reaction may be slowed down byadding relatively inert substances like an earth-alkaline carbonate oroxi e or a mixture of earth-alkaline carbonates or oxides. Forinstances, 5 grams of barium carbonate are used with grams of dioxide.The presence of a trace of water also slows down the retion.

The reaction of barium dioxide upon at least one of the molybdenum andtantalum metals, and possibly being mixed with other substances likebarium carbonate, may be effected outside the electrode cavity. Thepreferred conditions for this reaction are similar to those indicatedabove, namely a heating above red-heat, which may last a few secondsonly, in an atmosphere which, at least at the end, is reducing ornonoxidizing. This reaction gives an emissive ma terial which can beutilized for the electrodes; after being thus prepared, this materialhas to be fixed on the portion of the electrode designed for supportingthe activating substance. Such fixating may be effected in quite asatisfactory manner by mixing the material obtained after grinding itwith an additional amount of barium dioxide, by placing this mixture onthe portion of the electrode to be activated and by heating this portionso as to bring about the melting of the additional barium dioxide.

Figure 6 shows, by way of example, the re sults of tests made forstudying the variation, as a function of the value of the dischargecurrent, of the voltage at the terminals of fluorescent lamps cms. longand having an inside diameter of 15 mm.

The points of curve 38 were obtained with a lamp provided with twoso-called cold electrodes, nonactivated, of nickel plated iron, eachhaving an area of 12 square centimeters. Those of curve 39 are relativeto a lamp provided with electrodes having the same shape and arrangementas the one shown in Figure 3, but activated by means of a mixturecontaining, per electrode, about 60 mg. of barium dioxide and 40 mg. oftantalum powder. In the present case, each electrode is 4 mm. indiameter and 5 mm. long.

It may be seen from curve 39, that with the electrodes of the invention,the lamp terminal voltage remains substantially lower than that obtainedwith the cold electrodes and increases little when the currentdecreases, as long as the latter remains above inilliamperes. Thecathode drop of these electrodes is small even with currents as low as25 milliamperes; it is of the order of 25 volts (18 volts at 60milliamperes, 15 volts at 100 milliamperes). This is a particularlyremarkable result since, heretofore, it had not been found possible tooperate an electrode with a low voltage drop for currents below 100milliamperes.

A compariso of curves 39 and 38 show that the voltage at the terminalsof the lamp with electrodes according to the invention is lower by about200 volts than that of the lamp with cold electrodes. This difference isdue to the electrodes, since the latter constitute the only diiferencebetween the two lamps. This voltage gain makes it possible, forinstance, to supply in series two lamps of the type corresponding tocurve 39 with a transformer with a magnetic leakage having an opencircuit voltage of 1,300 volts and a normal current of 50 milliamperes;this transformer is normally used for supplying a single tubecorresponding to curve 38, having the same dimensions and the same lightpower as each one of the above tubes.

Another consequence of the low voltage drop of the electrodes accordingto the invention is that when a lamp provided with these electrodes,operates under 50 milliamperes, the temperature of its envelope reaches,at the electrode level, only '75 to 80 C., and only over less than 1 cm.length, although these electrodes are so-called hot electrodes; on thecontrary, for a similar lamp operatingalso under 50 milliamperes, butprovided with sc-called cold electrodes, the corresponding portion ofthe envelope reaches about 120 C. and measures about 4 cms. in length.

Another advantage of the electrodes according to the invention withrespect to cold electrodes is their very small bulk. The electrodesaccording to the invention make it possible to ensure that the lamp onwhich they are mounted is luminous over its entire length, especiallywhen the apertures of the open ends of the electrcdes face the stemwhich supports them, as represented in Figure 3. By mounting such lampsend to end, light source may be constituted as large as desired, andoffering practically no solution of continuity; on the contrary, of itis desired to obtain the same result with cold electrode lamps, saidelectrodes have to be housed in tubes of corresponding lengths which arewelded to the ends of the tube constituting the visible portion of thelamp and brought back parallel with the latter.

What we claim is:

l. A processs for manufacturing an emissive material for electrodes ofelectric discharge devices, comprising reacting barium dioxide with atleast one metal selected from the group consisting of molybdenum andtantalum at a temperature above 525 C. and lower than about 1,300 C.,said reaction taking place, at least at the end of its duration, in anon-oxidizing atmosphere at about the air pressure.

2. A process for manufacturing an emissive material for electrodes ofelectric discharge devices, comprising reacting barium dioxide with atleast one metal selected from the group consisting of molybdenum andtantalum at a temperature above 525 C. and lower than about l,300 C.,said reaction taking place, at least at the end of its duration, in anon-oxidizing atmosphere at about the air pressure and being stoppedbefore metallic barium volatilizes.

3. A process for manufacturing an emissive material for electrodes ofelectric discharge devices, comprising reacting barium dioxide with atleast one metal selected from the group consisting of molybdenum andtantalum, and with water in an amount which is small relatively to theamount of barium dioxide, at a temperature above 525 C. and lower thanabout 1,300 C.. said reaction taking place, at least at the end of itsduration, in a non-oxidizing atmosphere at about the air pressure.

4. A process for manufacturing an activated hollow electrode forelectric discharge device, said electrode being provided with an innercoating of emissive material, comprising bringing barium dioxide insidethe cavity of the electrode and reacting said barium dioxide with atleast one metal selected from the group consisting of molybdenum andtantalum at a temperature above 525 C. and lower than about 1,300 0.,said reaction taking place, at least at the end of its duration, in anon-oxidizing atmosphere at about the air pressure.

5. A process for manufacturing an emissive material for electrodes ofelectric discharge devices, comprising mixing barium dioxide with atleast one alkaline-earth metal compound selected from the groupconsisting of oxides and carbonates and reacting said barium dioxidewith at least one metal selected from the group consisting of molybdenumand tantalum at a temperature above 525 C. and lower than about 1,300"0., said reaction taking place, at least at the end of its duration, ina non-oxidizing atmosphere at about the air pressure.

6. A process for manufacturing an electron emissive coated electrode foran electric discharge device comprising reacting barium dioxide with atleast one metal selected from the group consisting of molybdenum andtantalum at a temperature above 525 C. and lower than 1,300 (7., saidreaction taking place, at least at the end of its duration, in anon-oxidizing atmosphere at about air pressure, grinding the materialobtained through said reaction, mixing this material with an additionalamount of barium dioxide, melting said mixture, and bringing intocontact said mixture with a metal support for emissive material.

'7. A process as claimed in claim 6, wherein said mixture is in contactwith said support during the melting thereof.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,467,398 Schumacher Sept. 11, 1923 1,720,675 Hertz July 16,1929 1,809,229 Bartlett et al. June 9, 1931 2,012,237 Beck Aug. 20, 19352,084,172 Weiller June 15, 1937 2,375,808 Miller May 15, 1945

4. A PROCESS FOR MANUFACTURING AN ACTIVATED HOLLOW ELECTRODE FORELECTRIC DISCHARGE DEVICE, SAID ELECTRODE BEING PROVIDED WITH AN INNERCOATING OF EMISSIVE MATERIAL, COMPRISING BRINGING BARIUM DIOXIDE INSIDETHE CAVITY OF THE ELECTRODE AND REACTING SAID BARIUM DIOXIDE WITH ATLEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM ANDTANTALUM AT A TEMPERATURE ABOVE 525* C. AND LOWER THAN ABOUT 1,300* C.,SAID REACTION TAKING PLACE, AT LEAST AT THE END OF ITS DURATION, IN ANON-OXIDIZING ATMOSPHERE AT ABOUT THE AIR PRESSURE.